In EUV lithography apparatuses, reflective optical elements for the extreme ultra violet (EUV) or soft X-ray wavelength range (e.g. wavelengths between about 5 nm and 20 nm), such as photomasks or multilayer mirrors, are used for the lithographic processing of semiconductor components. Since EUV lithography apparatuses usually have a plurality of reflective optical elements, they must have the highest possible reflectivity in order to ensure sufficient overall reflectivity. Since a plurality of reflective optical elements are usually arranged in series in an EUV lithography apparatus, the slightest deterioration in reflectivity of each individual reflective optical element has a substantial effect on the overall reflectivity within the EUV lithography apparatus.
Reflective optical elements for the EUV and soft X-ray wavelength range, usually have multilayer systems. These are alternately applied layers of a material having a higher real part of the refractive index at the operating wavelength (also referred to as spacer) and a material having a lower real part of the refractive index at the operating wavelength (also referred to as absorber), wherein an absorber-spacer pair forms a stack. This essentially simulates a crystal, wherein its lattice planes correspond to the absorber layers, on which Bragg reflections occur. The thicknesses of the individual layers as well as of the repetitive stacks can be constant across the entire multilayer system, or they can vary depending on which reflection profile is to be achieved.
For operating wavelengths in the wavelength range between 5 nm and 12 nm, in particular, the maximum reflectivity theoretically achievable by multilayer systems is smaller than in the wavelength range from about 12 nm to 20 nm. Also, the bandwidth of the reflected radiation is substantially smaller. An additional problem is that in the materials hitherto frequently used, e.g. lanthanum as an absorber and boron or boron carbide as a spacer, strong intermixing of the individual layers, forming a mixed layer, for example, of lanthanum boride, occurs even at room temperature, in particular at the interface between the boron or boron carbide and the lanthanum. This leads to a significant reduction in both the actual maximum reflectivity and the reflected bandwidth. Since usually a plurality of reflective optical elements are arranged in series in an EUV lithography apparatus, even slight deteriorations in the maximum reflectivity and bandwidth of each of the individual reflective optical elements over their lifespans has a pronounced effect on the overall reflectivity.